Were they at 600 ft height or 600 ft altitude?
If any aircraft 'approaching MDA' commences a go-around, it may well go slightly below MDA. Which is not permitted unless visual criteria are met...
Hence the decision to go-around must be initiated before MDA so that the aircraft does not fly below MDA, whereas for a precision approach the DA is the last point at which the decision must be made and the loss of height converting to a go-around is allowed for. Thus if you fly a non-precision approach as though it was a precision approach, you must use a higher approach minimum than the non-precision MDA.
However, if you fly an increased RoD (not more than 400 ft/nm) where this is permitted, you will have time to level at MDA (but not go below) and then maintain this until either the MAP is reached or visual criteria are met.